Inflatable packers or bridge plugs have long been used in wellbore operations. An inflatable packer typically comprises a tubular base and a surrounding, inflatable elastomeric bladder or sleeve. Fluid passages within the tubular body allow fluids to contact the inflatable bladder and expand the bladder radially outwardly to effect sealing engagement with a borehole or well casing.
Since the elastomeric bladder is subjected to both expansion pressure and abrasion or cutting forces, it has been common to surround the exterior surface of the bladder with a plurality of peripherally overlapping, resilient reinforcing slats or ribs. There is generally sufficient overlap of such slats that upon expansion of the inflatable bladder, the slats remain as a surrounding armor protecting the bladder from abrasion and cuts while also preventing extrusion of the bladder elastomer between the slats in a localized area.
Because the slats cannot effect the sealing of the packer against a wellbore or casing, at least some portions of the reinforcing slats are surrounded by and may be bonded to an outer annular elastomeric cover or packing element which, upon expansion of the inflatable packer, comes into pressure sealing engagement with the wellbore or casing.
The outer sealing cover generally comprises either a single or a plurality of annular circumferential elastomeric pieces located on the outer surface of the reinforcing slats. When a single elastomeric piece is employed it may cover only a portion of the longitudinal length of the slats or, alternatively, it may cover the entire outer surface of the slats. Such single piece covers generally have a uniform thickness along their length, the thickness generally being substantial.
In a typical inflatable packer, the bladder is retained between two metal elements of an end fitting, one abutting the inner cylindrical surface of the bladder and the other abutting the outer cylindrical surface of the bladder. The bladder is held in compression between these metal elements. Additionally, one or both of these metal elements may be provided with circumferential ribs or teeth which act to further retain the bladder in position and resist expansion stresses which would tend to pull the bladder out from between the compressive grip of the retainer elements. These types of assembly are shown in U.S. Pat. Nos. 3,918,520, 3,542,127, 4,003,581 and 4,892,144.
One additional force tending to work against bladder retention is not considered in prior art designs. Typically, the end edges of the bladder are subjected to pressure forces exerted by the inflation fluid used to expand the bladder. This additional pressure acts to push the end of the bladder out of its retaining assembly and, when added to the expansion stresses pulling on the bladder within its retention system, results in a common cause of failure of the bladder to retain pressure.